Polyester film for metal sheet laminating, metal sheet laminated with this film, and metal vessel formed from this metal sheet

ABSTRACT

A polyester film for metal sheet laminating. The film comprises a blend of (I) polybutylene terephthalate or a polyester comprising it as the main ingredient with (II) polyethylene terephthalte or a polyester comprising it as the main ingredient, the contents of the polyester (I) and the polyester (II) being 80 to 40 wt % and 20 to 60 wt %, respectively. This film has a melting point of 200 to 223° C. attributable to the polyester (I) and a melting point of 230 to 256° C. attributable to the polyester (II). The film as a whole has an intrinsic viscosity of 0.75 or higher.

TECHNICAL FIELD

[0001] The present invention relates to a polyester film for metal sheetlaminating, a metal sheet laminated with the film, and a metal containerformed from the metal sheet. More particularly, the invention relates toa polyester film for metal sheet laminating and a metal sheet laminatedwith the film, which are suitable for production of a container by deepdrawing and ironing of the metal sheet, and to a metal container formedfrom the metal sheet.

BACKGROUND ART

[0002] It is a common practice to apply a solvent-type paint essentiallycontaining a thermosetting resin on interior and exterior surfaces of ametal can for prevention of corrosion. However, formation of a coatingfilm of the solvent-type paint requires heating at a high temperature,and entails evaporation of a large amount of a solvent, therebypresenting problems associated with operation safety and environment.Recently, coating the surfaces of the can with a thermoplastic resin hasbeen proposed as a solvent-free corrosion preventing method. Amongthermoplastic resins, polyesters are particularly excellent informability and heat resistance, so that polyester-based films for metalsheet laminating particularly suitable for applications to cans are nowunder development.

[0003] For coating a metal sheet with a film, a thermoplastic resin ismelt-extruded directly on the metal sheet, or a thermoplastic resin filmis heat-pressed on the metal sheet directly or with the intervention ofan adhesive. Particularly, the use of the thermoplastic resin film isadvantageous, because the resin can be easily handled with an excellentoperability and the resin film has a highly uniform thickness. Since theuse of the intervening adhesive is disadvantageous in terms ofenvironmental consideration and costs, the direct heat-press of the filmon the metal sheet is more advantageous and attractive.

[0004] A metal can coated with the thermoplastic resin film is producedby laminating a metal sheet such as a steel sheet or an aluminum sheet(including a metal sheet subjected to a surface treatment such asplating) with the thermoplastic resin film, and working the laminatedmetal sheet into the can.

[0005] The thermoplastic resin film for use in such an applicationshould simultaneously satisfy various property requirements. That is,the film is required: (a) to have an excellent heat laminatability withrespect to the metal sheet; (b) to ensure an excellent can workabilitywithout film separation and occurrence of cracks and pinholes in a canworking process; (c) to be free from embrittlement when being subjectedto a printing process and a retort sterilizing process after the canworking process, and when being stored for a long period of time; and(d) to be excellent in content taste and flavor preserving property.

[0006] Several types of polyester films including films imparted withheat laminatability and films formed of polyesters blended orcopolymerized with other components for improvement of the canworkability have been proposed as the thermoplastic resin film for metalsheet laminating.

[0007] (A) Films each formed of a copolymer of polyethyleneterephthalate (PET) and a second component, for example, are stated inJP-B-8-19245, JP-B-8-19246 and Japanese Patent Publication No. 2528204.

[0008] (B) Films each formed of a blend comprising 99 to 60 wt % of apolyester copolymer containing ethylene terephthalate as a mainrecurring unit and having a melting point of 210 to 245° C. and 1 to 40wt % of polybutylene terephthalate (PBT) or a copolymer thereof aredisclosed in Japanese Patent Publication No. 2851468, JP-A-5-186612 andJP-A-5-186613.

[0009] (C) A film comprising 99 to 60 wt % of a polyester copolymer and1 to 40 wt % of a polyester mainly containing butylene terephthalate andhaving a melting point of 180 to 223° C., and having an intrinsicviscosity of not lower than 0.68 and lower than 0.75 is disclosed inJP-A-10-316775.

[0010] (D) A polyester film having an intrinsic viscosity of not lowerthan 0.75 is disclosed in JP-B-7-84532.

[0011] (E) A film of a polyester containing an ethylene terephthalateunit in a proportion of 90 mol % and having a reduction viscosity of 0.8to 1.1 dl/g is disclosed in JP-A-11-279294.

[0012] In the case (A), the films formed of the PET copolymer each havea lower melting point and a lower crystallinity, whereby the heatlaminatability and the formability are improved. However, the films areliable to be embrittled when being subjected to a heat treatment and theretort sterilizing process after the can working. Therefore, the shockresistance is disadvantageously reduced.

[0013] In the case (B), the films each comprise a PBT-based resin,whereby the embrittlement resistance and the shock resistance after thecan working and the heat laminatability are improved. However, the heatlaminatability and the adhesion with respect to a metal areinsufficient. Therefore, the formability in a high-level workingprocess, particularly in a deep drawing and ironing process, isunsatisfactory.

[0014] In the case (C), the film has a low intrinsic viscosity (notlower than 0.68 and lower than 0.75), so that a deformation follow-upproperty for a severer deep-drawing and ironing process is insufficient.In addition, the crystallization characteristic of the film isunsatisfactory because the resin virtually contains the PBT component ina proportion of not greater than 40 wt %. Therefore, the resistance tothe retort process, the long-term storage stability after the processand the shock absorbing property are not always sufficient.

[0015] In the case (D), the use of the polyester film having anintrinsic viscosity of not lower than 0.75 is proposed. It is statedthat the film is effective for the retort resistance and the shockresistance and for prevention of the deterioration in the taste of thecontent.

[0016] More specifically, a homogeneous copolymer essentially containingPET, polyethylene naphthalate or polycyclohexanedimethyleneterephthalate is proposed as the polymer. The film for the can isrequired to have properties contradictory to each other, e.g., aheat-laminating property, a deformation follow-up property for a severerhigh-level working process, resistance to adhesion to a working jig andlong-term storage stability after the retort process, to satisfy recenthigh performance requirements. However, it is difficult for theaforesaid homogeneous polymer to satisfy all the property requirements.

[0017] In the case (E), the polyester film is proposed, which containsthe ethylene terephthalate unit in a proportion of at least 90 mol %,has a reduction viscosity of 0.8 to 1.1 dl/g, and is durable in adeep-drawing and ironing process and less susceptible to whitening whenbeing immersed in boiling water. The polyester film is imparted with aboiling water resistance by employing a polymer containing the ethyleneterephthalate unit having a higher resistance to heat and boiling waterin a proportion of not smaller than 90 mol %. The polyester film isimparted with the adhesion to a metal by introducing a copolymerizablecomponent in a proportion of not smaller than 3 moles and smaller than10 moles into the polymer.

[0018] However, this polymer also falls within the category of thehomogeneous polymer, suffering from the same limitations as in the case(D). Therefore, the polymer cannot sufficiently satisfy the requirementfor the deformation follow-up property for a recent high-speed andhigh-level working process. More specifically, the concentration of thecopolymerizable component in the polyester should be increased ifgreater importance is placed on the deformation follow-up property. As aresult, the adhesion of the film to a jig is increased in the canworking process, thereby deteriorating the productivity, and the retortresistance and long-term storage stability of the can.

[0019] On the contrary, the inventors of the present inventionpreviously proposed biaxially stretched films each comprising 90 to 45wt % of a PBT or PBT-based polyester (I) and 10 to 55 wt % of a PET orPET-based polyester (II) (JP-A-9-194604, JP-A-10-110046). The films thusproposed each have a high crystallinity, and allow for heat-press at arelatively low temperature. In addition, the resulting laminated metalsheets are excellent in workability. Further, the films are notembrittled even during a retort sterilizing process and after long-termstorage, and are excellent in shock resistance.

[0020] Further, WO95-15993 proposes a polyester film composed of apolyester composition obtained by homogeneously mixing a polyethyleneterephthalate containing an ethylene terephthalate unit as a mainrecurring unit with a polybutylene terephthalate containing a butyleneterephthalate unit as a main recurring unit. The polyester film isdesigned so that the crystallization temperature, secondary transitiontemperature and plane orientation factor thereof respectively fallwithin predetermined ranges. Therefore, the polyester film is free fromwhite spots even if being subjected to the retort process after beingmelt-laminated on a metal sheet.

[0021] With recent increasing demands for a higher can producing rate, agreater can volume and a smaller can thickness, there is a tendencytoward an increase in the working deformation ratio of the metal in thedeep-drawing and ironing process, and toward an increase in frictionoccurring with respect to the working jig. Even if the film proposed bythe inventors is applied on a can body to be subjected to severedeformation, the film suffers from whitening or micro-cracks occurringdue to slight fluctuations in conditions for the production of thelaminated metal sheet or in conditions for the production of a final canproduct.

[0022] In addition, a residual strain occurring in the film due to anincreased working ratio may cause separation of the film due topartially insufficient adhesion to the metal, thereby arousing concernabout protection of the content of the can. Further, the film may adhereto a deep-drawing/ironing jig in the can production process, therebycausing rupture of the can body. Hence, it is demanded to improve thefilm so that the film is able to maintain its properties even underseverer working conditions. Since cans are extensively utilized for coldsoft drink, there is a great demand for improvement of the shockresistance of the laminate film to withstand an external impact forceexerted on the cans when the cans are dropped or during the processingor distribution of the cans. Since the cans are stored for a long periodof time, long-term stability is also an important property of the filmfor long-term cold storage of the cans and hot storage of the cans inwinter. As film-laminated cans are more extensively utilized, the filmand the film-laminated cans are now required to provide more advancedperformance. Hence, there is a demand for immediate development andimprovement of a film suitable for the film-laminated cans.

SUMMARY OF THE INVENTION

[0023] It is an object of the present invention to provide a polyesterfilm for metal sheet laminating, which is excellent in heatlaminatability with respect to a metal sheet, in can workability,particularly high-level workability for deep drawing and ironing, inshock resistance and in content taste and flavor preserving property,and is suitable for a film-laminated metal can, and to provide alaminated metal sheet and a metal container formed from the laminatedmetal sheet.

[0024] To achieve the aforesaid object, there is provided a polyesterfilm for metal sheet laminating, the polyester film comprising a blendof a polyester (I) consisting of polybutylene terephthalate orconsisting essentially of polybutylene terephthalate and a polyester(II) consisting of polyethylene terephthalate or consisting essentiallyof polyethylene terephthalate, the polyester (I) being present in thefilm in a proportion of 80 to 40 wt %, the polyester (II) being presentin the film in a proportion of 20 to 60 wt %, the polyester (I) having amelting point of 200 to 223° C., the polyester (II) having a meltingpoint of 230 to 256° C., the film, as a whole, having an intrinsicviscosity of not lower than 0.75.

[0025] With this arrangement, the polyester film for metal sheetlaminating can be provided which is excellent in heat laminatability, informability, particularly high-level workability for deep drawing andironing and in shock resistance and retort resistance after the working,and suitable for coating a metal can.

BRIEF DESCRIPTION OF THE DRAWING

[0026]FIG. 1 is a diagram illustrating peaks attributable to esterexchange in an NMR chart of a polyester film for metal sheet laminatingin accordance with the present invention.

DISCLOSURE OF THE INVENTION

[0027] The present invention will hereinafter be described in detail.

[0028] In the present invention, the polyester (I) consistingessentially of PBT means PBT or a copolymer of PBT and anothercomponent. In the film composed of the blend of the polyester (I) andthe polyester (II), the polyester (I) needs to have a melting point ofnot lower than 200° C. and not higher than 223° C. If the melting pointof the polyester (I) is lower than 200° C., the polyester has a lowercrystallinity, resulting in a lower heat resistance of the film.

[0029] Where the PBT copolymer is employed, the proportion and structureof the copolymerizable component may be selected so that the PBTcopolymer has a melting point in the aforesaid range. Thecopolymerizable component preferably comprises 1,4-butanediol in aproportion of not smaller than 80 mol %, particularly preferably notsmaller than 90 mol %, based on a total alcohol component. If theproportion of 1,4-butanediol is smaller than 80 mol %, the crystallinity(particularly the crystallization speed) is reduced, resulting inreduction in shock resistance and barrier property after a retortprocess.

[0030] The copolymerizable component is not particularly limited.Exemplary acids to be employed as the copolymerizable component includedicarboxylic acids such as isophthalic acid, phthalic acid,2,6-naphthalenedicarboxylic acid, 5-sodiumsulfoisophthalic acid, oxalicacid, succinic acid, adipic acid, sebacic acid, azelaic acid,dodecanedioic acid, dimer acids, maleic anhydride, maleic acid, fumaricacid, itaconic acid, citraconic acid, mesaconic acid andcyclohexanedicarboxylic acid, 4-hydroxybenzoic acid, ε-caprolactam andlactic acid.

[0031] Exemplary alcohols to be employed as the copolymerizablecomponent include ethylene glycol, diethylene glycol, 1,3-propanediol,neopentyl glycol, 1,6-hexanediol, cyclohexanedimethanol, triethyleneglycol, polyethylene glycol, polypropylene glycol, polytetramethyleneglycol, and ethylene oxide adducts of bisphenol A and bisphenol S.

[0032] Besides, three-functional group compounds such as trimelliticacid, trimesic acid, pyromellitic acid, trimethylolpropane, glycerol andpentaerythritol may be employed in a small amount. These compounds asthe copolymerizable component may be used in combination.

[0033] In the film according to the present invention, the polyester(II) consisting essentially of PET means PET or a copolymer of PET andanother component. In the film composed of the blend of the polyester(II) and the polyester (I), the polyester (II) needs to have a meltingpint of 230 to 256° C., preferably 236 to 256° C., more preferably 246to 256° C.

[0034] If the melting point is lower than 230° C., the crystallinity isreduced, and the resulting film suffers from whitening and white spotsafter the retort process and has a reduced shock resistance after theretort process. If the melting point is higher than 256° C., the heatlaminatability is reduced.

[0035] Particularly, where the polyester (II) has a melting point of notlower than 246° C., the heat resistance, the shock resistance after theretort process and the shock resistance after long-term storage areimproved. Further, troubles possibly occurring due to adhesion of thefilm to a jig in the can working process and troubles possibly occurringdue to rupture of the film during the working of a can body areeffectively suppressed.

[0036] The component copolymerizable with PET is not particularlylimited, but examples thereof include the same compounds as for thepolyester (I).

[0037] The film according to the present invention needs to entirelyhave an intrinsic viscosity (IV) of not lower than 0.75. If theintrinsic viscosity is lower than 0.75, the resulting film has aninsufficient practical performance, and is liable to be ruptured duringthe higher-level working of the can, leading to a remarkably reducedproductivity. Particularly, where the can has a greater volume, the filmis deformed at a greater deformation ratio when the laminated metalsheet is worked into the can through the deep-drawing and ironingprocess. The film cannot follow up the deformation, so that voids andcracks may occur in the film. Even a small external shock may promotethe separation of the film and the development of the cracks in thefilm. Where the film is used for coating the interior surface of thecan, the content of the can is brought into direct contact with themetal of the can. As a result, the taste and flavor preserving propertyis deteriorated, and a problem with flavor arises. Where the film isused for coating the exterior surface of the can, a print appearance ina portion of the film whitened due to the voids is deteriorated.Further, the voids and the cracks may cause corrosion of the can duringlong-term storage.

[0038] More specifically, the intrinsic viscosity is one indication ofthe molecular weight of the resin. A higher intrinsic viscosity means agreater molecular weight. In general, as the intrinsic viscosity or themolecular weight increases, the fluidity is reduced. Further, the heatlaminatability and the adhesion to the metal sheet are reduced, but thestrength and the shock resistance are improved. On the contrary, theinventive polyester film based on the particular PBT/PET composition ischaracterized in that the reduction in the heat laminatability and theadhesion to the metal sheet is suppressed even with an increasedintrinsic viscosity, and the strength and the shock resistance areincreased. In other words, the PBT/PET based film having an intrinsicviscosity of not lower than 0.75 has an excellent follow-up property fordeep drawing and ironing of the can body while maintaining its heatlaminatability and adhesion when the film is applied to a can as a metalcontainer.

[0039] In WO95/15993 described above, a polyester film for metal canlaminate-coating is disclosed which employs a PBT/PET based compositionas in the present invention. In WO95/15993, however, there is noteaching about the intrinsic viscosity. Since an object to be coated isa lid of a can, the film is not required to have a working follow-upproperty which is required when the film is applied to the can body asin the present invention. Accordingly, there is no statement about theworking follow-up property. In the present invention, the film isrequired to entirely have an intrinsic viscosity of not lower than 0.75.In WO95/15993, an intended crystallization characteristic is slightlyinfluenced by the intrinsic viscosity, but the crystallization speed isincreased with the reduction in the intrinsic viscosity. Therefore, alower intrinsic viscosity provides a more advantageous effect.

[0040] As the materials to be employed for the production of theinventive polyester film, the polyester (I) preferably has an intrinsicviscosity of not lower than 0.70, more preferably 0.75 to 1.6, and thepolyester (II) preferably has an intrinsic viscosity of not lower than0.60, more preferably 0.65 to 1.0. A melt mixture of the polyester (I)and the polyester (II) needs to have an intrinsic viscosity of not lowerthan 0.75, preferably not higher than 1.2.

[0041] If the intrinsic viscosities are higher than the aforesaidranges, a greater load is exerted on a melt extruder for melt-extrusionof the resin in the production of the film, so that the production speedmay be sacrificed. Further, the melted resin resides in the extruder foran excessively long period, so that the polyesters excessively reactwith each other. This deteriorates the properties of the film, resultingin deterioration of the properties of the metal sheet laminated with thefilm. Further, the higher intrinsic viscosities prolong polymerizationperiods and polymerization processes, thereby increasing the costs.

[0042] A polymerization method for the polyester materials is notparticularly limited, but examples thereof include an ester exchangemethod and a direct polymerization method. Exemplary catalysts for theester exchange include oxides and acetates of Mg, Mn, Zn, Ca, Li and Ti.Exemplary catalysts for the polycondensation include oxides and acetatesof Sb, Ti and Ge.

[0043] The polyester obtained through the polymerization may containmonomers, oligomers, side products such as acetaldehyde andtetrahydrofuran and the like. Therefore, solid phase polymerization ispreferably carried out at a temperature of not lower than 200° C. undera reduced pressure or in a stream of an inert gas.

[0044] Additives such as an anti-oxidation agent, a heat stabilizer, aUV absorber and an anti-static agent may be added to the polyestermaterials as required in the polymerization of the polyesters. Examplesof the anti-oxidation agent include hindered phenol compounds andhindered amine compounds. Examples of the heat stabilizer includephosphorus compounds. Examples of the UV absorber include benzophenonecompounds and benzotriazole compounds. As a reaction inhibitor forinhibiting a reaction between the different polyesters, a conventionallyknown phosphorus compound is preferably added before, during or afterthe polymerization. It is particularly preferred to add the reactioninhibitor after completion of the melt polymerization before the solidphase polymerization.

[0045] In the present invention, it is necessary that the ratio of thepolyester (I) to the polyester (II) in the film is (I)/(II)=80-40/20-60(wt %). Preferably, the ratio is (I)/(II)=70-55/30-45 (wt %).

[0046] If the proportion of the polyester (I) in the film is greaterthan 80 wt %, the high crystallinity characteristic of the polyester (I)is dominant, so that the workability of the film-laminated metal sheetis reduced and the shock resistance is deteriorated. If the proportionof the polyester (I) is smaller than 40 wt %, the crystallization speedis reduced, so that the properties of the film after the retort processare deteriorated.

[0047] Particularly where the proportion of the polyester (I) is in therange of 70 to 55 wt %, the working follow-up property is satisfactorywhen the laminated metal sheet is subjected to a high-speed andhigh-level deep-drawing and ironing process. Therefore, the film is freefrom whitening and micro-cracks which may otherwise occur due to voidsproduced in the film by forced deformation. In addition, the film hasexcellent adhesion to the metal sheet, so that the physical propertiesof the can after the retort process are well-balanced with the shockresistance of the can. Where the film is applied on the interior surfaceof the can, the can is excellent in corrosion resistance, contentprotecting property, taste and flavor preserving property and flavormaintaining property. Where the film is applied on the exterior surfaceof the can, the can is free from rust and imparted with a highly glossprint pattern, thereby providing a high quality product.

[0048] The concentration of terminal carboxyl groups in the inventivefilm is preferably not higher than 30 equivalents/ton. If theconcentration of the terminal carboxyl groups is higher than this level,the adhesion of the film to the metal jig is increased, and the slippageof the film is reduced in the can deep-drawing and ironing process.Therefore, the productivity is reduced, as the can production speed isincreased. Where the concentration of the terminal carboxyl groups ishigh, the film has a reduced surface hardness though the reason for thisis not known. Therefore, the film is easily scratched and separated inthe can working process, so that the metal can has a less gloss surface.In the worst case, a metal surface of the can is exposed. The exposureof the metal surface of the can reduces the corrosion resistance, anddeteriorates the taste and flavor preserving property with an influenceon the taste of food contained in the can. Further, the carboxyl groupspromote generation of lower molecular weight compounds in the film.These compounds may migrate into the food in the can, resulting indeterioration of the taste and flavor preserving property.

[0049] One method for controlling the concentration of the terminalcarboxyl groups is to prepare the material to be used by heating aprepolymer obtained through the melt-polymerization at a temperature ofnot lower than its glass transition temperature and not higher than itsmelting point and subjecting the prepolymer to the solid phasepolymerization in a stream of an inert gas or in vacuum. Other exemplarymethods include addition of a so-called terminal blocking agent whichreacts with the terminal carboxyl groups, reduction of a polymer melttemperature to a possible lowest level, and reduction of the moisturecontents of the polymers to possible lowest levels. These methods may beemployed in combination for the control of the concentration. However,the method for the concentration control is not limited to thesemethods, but any other methods which allow for substantial reduction ofthe concentration of the terminal carboxyl groups may be employed.

[0050] In the film according to the present invention, the index of theester exchange between the polyester (I) and the polyester (II) ispreferably 1 to 10%, more preferably 2 to 7% (as measured by a method tobe described later).

[0051] More specifically, the inventive film is composed of a blend ofPBT (BD-TPA-BD structural unit) and PET (EG-TPA-EG structural unit). ThePBT and the PET are melt-mixed with each other at a temperature notlower than the melting points thereof. At this time, an ester exchangereaction takes place at ester bonds (monomer links), whereby a BD-TPA-EGstructural unit which is not present in the resin materials isgenerated. The ester exchange index is a numerical expressionrepresenting the extent of the generation of this structural unit, whichvaries depending on the melt temperature, the melt period and the amountof the catalyst in the materials. As the ester exchange reactionproceeds, the glass transition temperature and the crystallinity arereduced. Further, the heat-up crystallization speed is drasticallyincreased.

[0052] If the glass transition temperature and the crystallinity arereduced, the dragging resistance is deteriorated in the can productionprocess. If the heat-up crystallization speed is increased, thecrystallization is promoted in the can working process, resulting inwhitening and rupture of the film.

[0053] In other words, where the ester exchange index is increased,particularly to higher than 10%, to further randomize the structuralunits of the polyesters (I) and (II), the melting point of the film isreduced and the heat resistance is deteriorated. Further, thecrystallization speed is drastically increased when the film isstretched in the working process, thereby deteriorating the formability.

[0054] On the contrary, if the ester exchange index is not higher than1%, the resulting film has a poorer deformation follow-up property, andthe resulting metal sheet has a poorer workability. This is because thepolyethylene terephthalate component maintains its properties while thehigh crystallinity PBT is present therein.

[0055] Where the ester exchange index falls within the preferred range,the crystallization speed is not excessively high. When the metal sheetis worked into the can, the film is prevented from adhering to theworking jig, and the friction is reduced. Therefore, the resulting canhas a more uniform surface.

[0056] A method for controlling the ester exchange index in theaforesaid range is not particularly limited, but the control of theester exchange index may be achieved by controlling the melt temperatureof the polyesters (I) and (II) in the extruder, the mixing degree in theextruder and the residence time in the extruder. A method formelt-mixing the PBT and the PET is not particularly limited, but themixing of the PBT and the PET may be achieved by melt-mixing chips ofthe materials preliminarily blended in a single extruder, or byseparately melting the materials in different extruders and then mixingthe melted materials. The latter method is preferred in terms of thecontrol of the ester exchange reaction. The ester exchange issignificantly influenced by the type and amount of the polymerizationcatalyst for the polyesters, and the residual activity of the catalyst.Therefore, it is important to properly select the type and amount of thecatalyst. Further, a catalysis inhibitor such as a phosphorus compoundmay also be employed.

[0057] In the film according to the present invention, a PBT residueindex is preferably 40 to 75% (as measured by a method to be describedlater). If the PBT residue index is smaller than 40%, the crystallinityof the film is reduced, so that the corrosion resistance and the tasteand flavor preserving property of the can are deteriorated. If the PBTresidue index is greater than 75%, the adhesion of the film is reducedafter the retort process, and the shock resistance is reduced.

[0058] The PBT residue index is a numerical expression representing theratio of the residue of the BD-TPA-BD unit after the melt-mixing of thePBT and the PET in view of the structural unit (herein the PBT) of thefilm forming resin, similarly to the ester exchange index. By definingthis numerical expression, an influence of the copolymerization degreeon the can workability is taken into consideration. If the meltingpoints of the materials respectively fall within the predeterminedranges, the copolymerization is possible. However, the BD-TPA-BD unitneeds to be present in the film in an amount not smaller than apredetermined level. If the PBT residue index is reduced, thecrystallinity of the film is reduced as described above.

[0059] With the same ester exchange index, the PBT residue index dependson the copolymerization degree of the film material. Where the PBTcopolymer is employed as the film material, the ester exchange should besuppressed more than in the case where the PBT not copolymerized isemployed. Thus, the reaction occurring during the melt-mixing isrelatively suppressed.

[0060] In the film according to the present invention, a heat-upcrystallization peak temperature (Tc) at which a transition from anamorphous phase occurs is preferably 60 to 100° C., more preferably 60to 90° C.

[0061] If the Tc is higher than 100° C., the film is embrittled in theretort sterilizing process, and white spots occur in the film todeteriorate the appearance of the film. If the Tc is lower than 60° C.,the formability at a higher working temperature and the content tasteand flavor preserving property are deteriorated.

[0062] In the film according to the present invention, a heat-upcrystallization index (Cp) is preferably not smaller than 0 J/g•° C. ina temperature range of 60 to 100° C.

[0063] If the Cp is smaller than 0 J/g•° C., the crystallization speedis excessively high around a temperature at which the crystallizationbegins, so that the formability is deteriorated. Therefore, the film isliable to suffer from whitening, pinholes and cracks in the high-levelworking process such as the deep-drawing and ironing process. The canworking process is generally performed in the temperature range of 60 to100° C., and it is important that the Cp is not smaller than 0 J/g•° C.in this temperature range.

[0064] The Cp serves as an indication of the crystallization speedobserved when the film is heated up for transition from the amorphousphase. The Cp corresponds to the amount of heat released from the filmwhen the film temperature is increased by one degree during thecrystallization. The crystallization speed increases, as the heat-upcrystallization index Cp reduces. The value of the Cp depends on thecrystallization characteristics, the viscosity, the ester exchange indexand the IV of the material resin, and the types and amounts of theadditives.

[0065] When the PBT and the PET are melt-mixed at a higher melttemperature or under higher shearing conditions for an extended periodof time, the ester exchange reaction and a decomposition reactionproceed, so that the characteristics of the mixture are drasticallychanged. Particularly where the ester exchange excessively proceeds, themelting point and the glass transition temperature are reduced, and theheat-up crystallization index is reduced below 0 J/g•° C. As a result,the excellent characteristics of the film provided by the polyester (I)and the polyester (II) may be lost, so that the heat resistance and theformability are deteriorated.

[0066] The heat-up crystallization index (Cp) is significantlyinfluenced by the ester exchange index as well as by the IV. If the IVis lower, the crystallization speed is higher. Since the additives inthe film forming resin serve as crystalline nuclei, the heat-upcrystallization index is also influenced by the types and amounts of theadditives.

[0067] In the film according to the present invention, thecrystallization speed at a temperature around the Tg is controlled byblending the PBT having a very high crystallization speed and the PEThaving an extremely low crystallization speed at a temperature aroundthe Tg in a predetermined state. That is, the film is crystallized at atemperature around the Tg, but the crystallization speed is controlledat a relatively low level and the crystallization degree is high inaccordance with the present invention. The film has such uniquecharacteristics which cannot generally be provided by a homogeneouspolymer system.

[0068] Further, the film according to the present invention preferablyhas an elongation of not smaller than 100% at rupture at the heat-upcrystallization peak temperature. If the elongation is smaller than100%, the can workability is deteriorated.

[0069] For production of the film according to the present invention,the polyesters (I) and (II) are blended in a proper ratio, andmelt-mixed at a temperature of 250 to 280° C. for 3 to 15 minutes bymeans of an extruder. Then, the melt mixture is extruded through a T-dieinto a sheet form. The sheet is brought into intimate contact with acooling drum maintained at a temperature not higher than a roomtemperature so as to be cooled. Thereafter, the resulting unstretchedfilm is introduced into a simultaneous biaxial stretching machine, andbiaxially stretched at a draw ratio of 2 to 4 in a machine direction(MD) and in a transverse direction (TD) at a temperature of 50 to 150°C. Further, the film is subjected to a heat treatment at 80 to 220° C.for several seconds at a TD relaxing ratio of several percent. Thus, theintended film is produced. Prior to the introduction of the film intothe simultaneous stretching machine, the film may preliminarily bestretched longitudinally at a draw ratio of about 1 to about 1.2.

[0070] The film may be produced by a successive stretching method. To bebriefly described, the unstretched film is heated with the use of a rollheater or an infrared radiation, and stretched longitudinally to providea longitudinally stretched film. The stretching is carried out in atemperature range between the glass transition temperature (Tg) of thepolyester and a temperature higher by 40 degrees than the Tg byutilizing a difference between circumferential speeds of two or morerolls. The draw ratio is preferably not smaller than 2.5 and not higherthan 3.6. Then, the longitudinally stretched film is sequentiallysubjected to a transverse stretching process, a heat setting process anda heat relaxation process to provide a biaxially oriented film. Thetransverse stretching process is preferably started at a temperature inthe range between the Tg of the polyester and the temperature higher by40 degrees than the Tg. The maximum temperature for the transversestretching is preferably a temperature lower by 100 to 40 degrees thanthe melting point (Tm) of the polyester. The draw ratio for thetransverse stretching is adjusted depending on physical propertyrequirements of the final film, but is typically not smaller than 2.7,preferably not smaller than 3.0, more preferably not smaller than 3.6.In the heat setting process following the stretching process, the filmmay be stretched by 2 to 20% along the width thereof, but the stretchratio is preferably included in the total draw ratio. After the heatsetting process, the film is subjected to a process (called “relaxationprocess”) for continuously reducing the width thereof to control theheat shrinkability of the film. Then, the film is cooled to not higherthan the Tg to provide a biaxially stretched film.

[0071] The heat treatment after the stretching is required for impartingthe film with dimensional stability, and may be achieved by a knowmethod, e.g., by application of hot air, by application of infraredradiation, or by application of microwave. Among these methods, the hotair application method is most preferred because the film can evenly beheated with a high level of accuracy.

[0072] To facilitate the passage of the film and the sheet in the filmproduction process and the can production process, a small amount of aninorganic lubricant such as silica, alumina or kaolin is preferablyadded to the material prior to the film production to make the surfaceof the film more slippery. For improvement of the appearance andprintability of the film, the film may contain, for example, a siliconecompound

[0073] The inorganic lubricant is preferably present in the film in aproportion of 0.001 to 0.5 wt %, more preferably 0.05 to 0.3 wt %.Further, titanium dioxide may be added to the material in a proportionup to about 20% for opacification as well as lubrication. Even withaddition of greater than 40% of titanium dioxide, the stretched film canbe obtained in the simultaneous biaxial stretching process.

[0074] A metal sheet such as a steel sheet or an aluminum sheet isheat-laminated with the inventive polyester film. Preferably used as themetal sheet to be laminated is a steel sheet subjected to a chemicaltreatment such as a chromic acid treatment, a phosphoric acid treatment,an electrolytic chromic acid treatment or a chromate treatment, or aplating process employing nickel, tin, zinc, aluminum, a gun metal or abrass.

[0075] For further improvement of heat-bondability and subsequentadhesion of the film to the metal sheet, an adhesive layer may beprovided on the film by co-extrusion, laminating or coating. Theadhesive layer preferably has a thickness of not greater than 5 μm, morepreferably not greater than 1 μm, on a dry basis. The adhesive layer isnot particularly limited, but is preferably a thermosetting resin layersuch as composed of an epoxy resin, a polyurethane resin or a polyesterresin, or a resin obtained by modifying any of these resins.

[0076] For improvement of the exterior appearance and printability ofthe metal can and the heat resistance and the retort resistance of thefilm, one or more types of resin layers may be provided on a surface ofthe film opposite from the surface heat-bonded to the metal sheet. Theprovision of these resin layers may be achieved by co-extrusion,laminating or coating.

[0077] When the metal sheet is laminated with the inventive film, themetal sheet is preheated at 160 to 250° C., and press-bonded onto thefilm by means of rolls maintained at a temperature lower by at least 30degrees, preferably at least 50 degrees, than the temperature of themetal sheet. Then, the laminated metal sheet is cooled to a roomtemperature. Thus, the metal sheet laminated with the film iscontinuously produced.

[0078] The preheating of the metal sheet is achieved by a heater-rollheat conduction method, an inductive heating method, a resistive heatingmethod or a hot air heat transfer method. Particularly, the heater-rollheat conduction method is preferred in consideration of simplificationand cost reduction of the facility.

[0079] For the cooling after the laminating process, the laminated metalsheet is immersed in a coolant such as water, or brought into contactwith a cooling roll.

[0080] The metal sheet thus obtained may be subjected to a workingprocess as it is. Alternatively, the metal sheet may be subjected to aheat treatment at a temperature higher by 10 to 30 degrees than themelting point of the polyester, and then rapidly cooled. Thus, thepolyester film is brought into an amorphous phase, so that the laminatedmetal sheet has a higher workability.

[0081] In other words, the workability of the can (metal container) issignificantly influenced by the crystallizability of the film in theamorphous phase. More specifically, the metal sheet laminated with thepolyester film is deformed or drawn into a cylindrical shape or amodified cylindrical shape, and then subjected to an ironing process inthe can working process. At this time, the surface of the polyester filmbonded to the metal sheet is often in the amorphous phase or in a nearlyamorphous phase. Particularly where the film is heat-bonded to the metalsheet without the intervention of the adhesive, the film has a higheramorphous degree. Further, a part or all of the resin is brought intothe amorphous phase for improvement of the deep-drawing and ironingworkability. In a known technique, it is difficult to satisfy aformability requirement for a severe deep-drawing and ironing process aswell as a quality requirement for the shock resistance and the retortresistance of the can. However, as described above, the presentinvention focuses on the heat-up crystallization peak temperature andthe heat-up crystallization index observed when the film is in theamorphous phase, making it possible to satisfy both of the aforesaidrequirements.

[0082] A metal container or a can which is processed to be ready tocontain food or drink therein may be named as the metal container. In abroad sense, the metal container according to the present inventionincludes a part of the metal container, for example, a can lidconfigured to be ready for a crimping process.

[0083] Where the laminated metal sheet is employed for a can body memberof a three-piece can (3P-can) to be produced through a severe neckingprocess or for a can body member of a two-piece can (2P-can) to beproduced through a deep-drawing and ironing process, the excellentworkability of the inventive film is particularly advantageous.

[0084] The metal can (metal container) produced with the use of theinventive film is suitable for containing coffee, green tea, red tea,oolong tea and other various process food because of its excellentretort resistance, flavor preserving property and corrosion resistance.

EXAMPLES

[0085] Next, the present invention will be described more specificallyby way of examples thereof.

[0086] Materials for films according to Examples and ComparativeExamples and methods for measuring physical property values are asfollows:

[0087] (1) Materials

[0088] Polyester (I)

[0089] A-1: PBT subjected to solid phase polymerization, having an IV of1.40 dl/g and a Tm of 223° C., and containing 40 ppm of a Ti catalystand 5 equivalents/ton of COOH groups.

[0090] A-2: PBT subjected to solid phase polymerization, having an IV of1.22 dl/g and a Tm of 223° C., and containing 40 ppm of a Ti catalystand 7 equivalents/ton of COOH groups.

[0091] A-3: PBT subjected to solid phase polymerization, having an IV of1.08 dl/g and a Tm of 223° C., and containing 40 ppm of a Ti catalystand 7 equivalents/ton of COOH groups.

[0092] A-4: PBT subjected to solid phase polymerization, having an IV of0.94 dl/g and a Tm of 223° C., and containing 100 ppm of a Ti catalystand 12 equivalents/ton of COOH groups.

[0093] A-5: PBT not subjected to solid phase polymerization, having anIV of 0.90 dl/g and a Tm of 223° C., and containing 40 ppm of a Ticatalyst and 35 equivalents/ton of COOH. groups.

[0094] A-6: PBT subjected to solid phase polymerization, having an IV of0.80 dl/g and a Tm of 223° C., and containing 40 ppm of a Ti catalystand 15 equivalents/ton of COOH groups.

[0095] A-7: PBT not subjected to solid phase polymerization, having anIV of 0.65 dl/g and a Tm of 223° C., and containing 100 ppm of a Ticatalyst and 50 equivalents/ton of COOH groups.

[0096] A-8: A copolymer of PBT subjected to solid phase polymerizationand 5 mol % of sebacic acid (SEA), having an IV of 0.92 dl/g and a Tm of217° C., and containing 40 ppm of a Ti catalyst and 18 equivalents/tonof COOH groups.

[0097] A-9: A copolymer of PBT not subjected to solid phasepolymerization and 12 mol % of SEA, having an IV of 0.95 dl/g and a Tmof 204° C., and containing 30 ppm of a Ti catalyst and 30equivalents/ton of COOH groups.

[0098] A-10:A copolymer of PBT subjected to solid phase polymerizationand 5 mol % of isophthalic acid (IPA), having an IV of 1.05 dl/g and aTm of 216° C., and containing 40 ppm of a Ti catalyst and 23equivalents/ton of COOH groups.

[0099] Polyester (II)

[0100] B-1: PET subjected to solid phase polymerization, having an IV of0.90 dl/g and a Tm of 255° C., and containing 40 ppm of a Ge catalystand 10 equivalents/ton of COOH groups.

[0101] B-2: PET subjected to solid phase polymerization, having an IV of0.75 dl/g and a Tm of 255° C., and containing 40 ppm of a Ge catalystand 15 equivalents/ton of COOH groups.

[0102] B-3: PET subjected to solid phase polymerization, having an IV of0.64 dl/g and a Tm of 255° C., and containing 100 ppm of a Sb catalystand 20 equivalents/ton of COOH groups.

[0103] B-4: PET not subjected to solid phase polymerization, having anIV of 0.62 dl/g and a Tm of 255° C., and containing 100 ppm of a Sbcatalyst and 50 equivalents/ton of COOH groups.

[0104] B-5: A copolymer of PET subjected to solid phase polymerizationand 50 mol % of IPA, having an IV of 0.81 dl/g and a Tm of 243° C., andcontaining 100 ppm of a Sb catalyst and 18 equivalents/ton of COOHgroups.

[0105] B-6: A copolymer of PET not subjected to solid phasepolymerization and 12 mol % of IPA, having an IV of 0.65 dl/g and a Tmof 226° C., and containing 100 ppm of a Sb catalyst and 50equivalents/ton of COOH groups.

[0106] B-7: A copolymer of PET subjected to solid phase polymerizationand 5 mol % of SEA, having an IV of 0.78 dl/g and a Tm of 239° C., andcontaining 100 ppm of a Sb catalyst and 25 equivalents/ton of COOHgroups.

[0107] (2) Measuring Methods

[0108] A. Intrinsic Viscosity (IV)

[0109] A solution with a concentration of 0.5 g/dl was prepared with theuse of a solvent mixture containing phenol and tetrachloroethane inequivalent weights, and the intrinsic viscosity was determined on thebasis of the viscosity of the solution measured at a temperature of 20°C.

[0110] B. Ester Exchange Index (Ex) and PBT Residue Index (Ea)

[0111] A 13C NMR measurement was performed by means of a nuclearmagnetic resonance apparatus GEMINI2000/300 (magnetic field strength:7.05 T) available from Varian Inc. A sample for the measurement wasprepared by dissolving a 60- to 100-mg film in 0.7 ml of a CF₃COODsolvent. The ester exchange index (Ex) was determined on the basis ofvalues obtained through integration of peaks (FIG. 1) attributable tothe ester exchange from the following expression:

Ex=(Sab+Sba)/(Saa+Sbb+Sab+Sba)×100(%)

[0112] Similarly, the PBT residue index (Ea) was determined from thefollowing expression:

Ea=Saa/(Saa+Sbb+Sab+Sba)×100(%)

[0113] C. Melting Point (Tm) and Heat-up Crystallization PeakTemperature (Tc)

[0114] The melting point (Tm) and the heat-up crystallization peaktemperature (Tc) were measured by means of a DSC available fromPerkin-Elmer Corp., while a film sample was heated up at 200° C./min.The film sample for the measurement was prepared by melting a stretchedfilm and then rapidly cooling the film at a rate of not lower than 100°C./min to bring the film into an amorphous phase.

[0115] D. Heat Laminatability

[0116] A 0.21-mm thick tin-free steel sheet and a film sample stackedthereon were supplied at a rate of 20 m/min between a metal roll heatedat a predetermined temperature and a silicone rubber roll so as to beheat-bonded to each other with a linear pressure of 4.9×10⁴ N/m and,after a lapse of two seconds, immersed in ice water for cooling. Thus, alaminate was obtained as a laminated metal sheet.

[0117] Then, eleven 18-mm wide test strips (each having an unlaminatedportion at an end thereof and a laminated portion having an MD length ofnot smaller than 8 cm) were cut out of the laminate in the TD direction.

[0118] Then, an adhesive tape as specified by JIS Z-1522 was appliedonto a film surface of each of the test strips, and a 180-degree peeltest was performed at a rate of 10 mm/min by means of an autographavailable from Shimadzu Corp. for measurement of a peel strength. Theadhesion of the test strip was evaluated on the following criteria:

[0119] ⊚ (excellent laminatability): Ten or more of the test strips hada peel strength of not smaller than 2.9 N, or film rupture occurredtherein with a force of not smaller than 2.9 N.

[0120] ◯ (good laminatability): Five to nine of the test strips had apeel strength of not smaller than 2.9 N, or film rupture occurredtherein with a force of not smaller than 2.9 N.

[0121] E. Formability

[0122] The laminated metal sheet prepared in the test D was subjected toa deep-drawing and ironing process for production of a two-piece canhaving a volume of 500 ml with an interior surface of a can body thereofbeing defined by the film of the laminated metal sheet. Then, the canthus produced was filled with 1-wt % salt water, and an electric currentwas measured with a voltage of 6 V being applied to the can body as ananode, whereby an evaluation was made on defects in the polyester film.In general, the defects increase as the electric current increases. Theelectric current is preferably not greater than 1 mA to ensure thequality of the can. Where the electric current was not smaller than 5mA, the laminated metal sheet was regarded to have an unacceptableformability (indicated by ×).

[0123] F. Retort Resistance

[0124] The laminated metal sheet prepared in the test D was subjected toa retort process at 120° C. for 30 minutes, and then the state of thefilmwas inspected. In the evaluation, a laminated metal sheet whichapparently suffered from whitening or white spots was regarded to havean unacceptable retort resistance (indicated by ×), and a laminatedmetal sheet which suffered from whitening not apparently but visuallyperceivably was regarded to have a poor retort resistance (indicated byΔ). Further, a laminated metal sheet which experienced no change asvisually observed was regarded to have an excellent retort resistance(indicated by ◯).

[0125] G. Shock Resistance

[0126] Test samples were prepared from the laminated metal sheetsprepared in the test D. A set (i) of ten test samples were subjected toa retort process at 125° C. for 30 minutes. A set (ii) of ten testsamples were subjected to a retort process at 125° C. for 30 minutes andthen stored in a 50° C. atmosphere for one month. Then, a 1-kg weight(having a spherical front end having a diameter of ½ inch) was droppedonto the film surface of each of these test samples from a height of 50cm in a 5° C. atmosphere, and the state of the film was observed. Theshock resistance was evaluated on the following criteria:

[0127] × (Unacceptable): One or more of the test samples suffered fromthe separation or rupture of the film as visually observed.

[0128] Δ (Poor): None of the test samples experienced the separation andrupture of the film as visually observed, but three or more of the testsamples suffered from the corrosion of the metal when being immersed inan aqueous solution of copper sulfate.

[0129] ◯ (Good): None of the test samples experienced the separation andrupture of the film as visually observed, but two or less of the testsamples suffered from the corrosion when being immersed in an aqueoussolution of copper sulfate.

[0130] ⊚ (Excellent): None of the test samples experienced theseparation and rupture of the film as visually observed, and none of thetest samples suffered from the corrosion when being immersed in anaqueous solution of copper sulfate.

[0131] H. Taste and Flavor Preserving Property

[0132] The 500-ml 2P-can body obtained in the test E was filled with500-g distilled water, and a commercially available 202 diameteraluminum EO lid was crimped on the can body to seal the can body. Theresulting can was subjected to the retort process at 125° C. for 30minutes. Then, the can was sufficiently cooled to a room temperature,and a tasting test on the content of the can was carried out by 50panelists to judge if there is a taste difference between the content ofthe can and the distilled water. On the basis of the results of thetasting test, the taste and flavor preserving property was evaluated onthe following criteria. However, the film regarded to have anunacceptable formability (indicated by ×) in the formability evaluationin the test E was not subjected to the tasting test, but judged to havean unacceptable taste and flavor preserving property.

[0133] ◯ (Good): Five or less of the panelists recognized thedifference.

[0134] Δ (Poor): Not less than five and less than ten of the panelistsrecognized the difference.

[0135] × (Unacceptable): Ten or more of the panelists recognized thedifference.

[0136] I. Heat-up Crystallization Index (Cp)

[0137] By means of a DSC available from Perkin-Elmer Corp., the heat-upcrystallization index was measured in conformity with JIS K7123-1987.Sapphire was used as a reference substance. A sample for the measurementwas prepared by melting a stretched film and then rapidly cooling thefilm at a rate of not lower than 100° C./min to bring the film into anamorphous phase. Where the heat-up crystallization peak temperature (Tc)did not fall within a temperature range of 60 to 100° C., a minimumvalue observed in the range of 60 to 100° C. was employed as the heat-upcrystallization index (Cp).

[0138] J. Elongation (%) in Tension

[0139] With the use of film samples (n=5) each having a width of 10 mmand a length of 10 cm, the elongation in tension was measured at theheat-up crystallization peak temperature (Tc) in conformity with ASTMD882. Data is expressed by minimum values of MD elongation and TDelongation.

[0140] (3) Examples and Comparative Examples

[0141] Examples 1 to 6 and Comparative Examples 1 to 10

[0142] Unstretched films were each produced by blending a polyester (I)and a polyester (II) each having acomposition as shown in Table 1,adding 0.1 wt % of silica having an average particle diameter of 1.1 μmto the blend, melt-mixing the blend by means of an extruder, extrudingthe resulting melt through an outlet of a T-die, and rapidly solidifyingthe extruded melt.

[0143] Subsequently, the unstretched films were each transported througha preheating zone at 60° C. with edges thereof being held by clips of atenter-type simultaneous biaxial stretching machine, and thensimultaneously biaxially stretched at an MD draw ratio of 3.0 and at aTD draw ratio of 3.3. Thereafter, the resulting films were eachsubjected to a heat treatment at 150° C. at a TD relaxing ratio of 5%for 4 seconds, then cooled to a room temperature, and wound up. Thus,25-μm thick biaxially stretched films were obtained.

[0144] With the use of each of the films thus obtained, a laminatedmetal sheet was produced by the method described in the test D, and thenevaluated. Further, the formability of the film of the laminated metalsheet produced by the method described in the test D was evaluated inthe manner described in the test E. Furthermore, the retort resistance,the shock resistance and the taste and flavor preserving property of thelaminated metal sheet were evaluated in the manner described in thetests F, G and H, respectively. The physical properties of the films andthe results of the evaluations are shown in Table 2. TABLE 1 Example 1 23 4 5 6 7 8 9 10 11 12 Polyester I Composition A-4 A-2 A-1 A-3 A-8 A-5A-4 A-2 A-8 A-3 A-3 A-10 Intrinsic viscosity 0.94 1.22 1.4 1.08 0.92 0.93.94 1.22 0.92 1.08 1.08 1.05 COOH-group (eq/ton) 12 7 5 7 18 35 12 7 187 7 23 Tm (° C.) 223 223 223 223 217 223 223 223 217 223 223 216Polyester II Composition B-3 B-2 B-2 B-2 B-3 B-5 B-3 B-2 B-3 B-2 B-2 B-2Intrinsic viscosity 0.64 0.75 0.75 0.75 0.64 0.81 0.64 0.75 0.64 0.750.75 0.75 COOH-group (eq/ton) 20 15 15 15 20 18 20 15 20 15 15 15 Tm (°C.) 255 255 255 255 255 243 255 255 255 255 255 255 Mixing conditionsI/II (wt %) 50/50 55/45 60/40 75/25 60/40 50/50 50/50 55/45 60/40 50/5060/40 75/25 Extrusion temp (° C.) 270 275 280 275 270 285 270 275 270275 270 270 Residence time (min) 6 6 6 5 6 8 6 6 6 8 5 8 Stretchingmethod *1 *1 *1 *1 *1 *1 *2 *2 *2 *1 *1 *1 Stretchability Good Good GoodGood Good Good Good Good Good Good Good Good Film producing conditionsMD temp (° C.) 80 80 80 80 80 80 55-60 55-60 55-60 80 80 80 Ratio 3 3 33 3 3 2.8 2.8 2.8 3 3 2.8 TD temp (° C.) 80 80 80 80 80 80 80-90 80-9080-90 80 80 80 Ratio 3.3 3.3 3.3 3.3 3.3 3.3 3.6 3.6 3.6 3.3 3.3 3 Heatsetting 150 150 150 150 150 150 150 150 150 150 150 150 temp (° C.)Thickness (μm) 25 25 25 25 25 25 25 25 25 25 25 20 Example ComparativeExample 13 1 2 3 4 5 6 7 8 9 10 11 Polyester I Composition A-3 A-2 A-4A-6 A-4 — A-3 A-5 A-8 A-9 A-7 A-4 Intrinsic viscosity 1.08 1.22 0.94 0.80.94 1.08 0.9 0.92 0.95 0.65 0.94 COOH-group (eq/ton) 7 7 12 15 12 7 3518 30 50 12 Tm (° C.) 223 223 223 223 223 223 223 217 204 223 223Polyester II Composition B-7 B-2 B-3 B-3 B-3 B-5 B-5 B-6 B-4 B-2 B-1 B-3Intrinsic viscosity 0.78 0.75 0.64 0.64 0.64 0.81 0.81 0.65 0.62 0.750.9 0.64 COOH-group (eq/ton) 25 15 20 20 20 18 18 50 50 15 10 20 Tm (°C.) 239 255 255 255 255 243 243 226 255 255 255 255 Mixing conditionsI/II (wt %) 60/40 35/65 85/15 50/50 50/50 0/100 30/70 60/40 60/40 50/5055/45 85/15 Extrusion temp (° C.) 260 280 280 275 280 270 270 275 275275 280 280 Residence time (min) 10 8 6 6 10 6 6 6 8 6 8 6 Stretchingmethod *1 *1 *1 *1 *1 *1 *1 *1 *1 *1 *1 *2 Stretchability Good Good GoodGood Good Good Good Good Good Good Good Bad Film producing conditions MDtemp(° C.) 80 80 80 80 80 80 80 80 80 80 80 55-60 Ratio 3 3 3 3 3 3 3 33 3 3 2.8 TD temp(° C.) 80 80 80 80 80 80 80 80 80 80 80 80-90 Ratio 3.33.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.6 Heat setting 150 150 150 150150 150 150 150 150 150 150 150 temp(° C.) Thickness (μm) 25 25 25 25 2525 25 25 25 25 25 25 Comparative Example 12 13 14 15 16 17 Polyester IComposition A-3 A-8 A-3 A-4 A-3 A-4 Intrinsic viscosity 1.08 0.92 1.080.94 1.08 0.94 COOH-group (eq/ton) 7 18 7 12 7 12 Tm (° C.) 223 217 223223 223 223 Polyester II Composition B-5 B-4 B-2 B-3 B-2 B-3 Intrinsicviscosity 0.81 0.62 0.75 0.64 0.75 0.64 COOH-group (eq/ton) 18 50 15 2015 20 Tm (° C.) 243 255 255 255 255 255 Mixing conditions I/II (wt %)30/70 60/40 65/35 60/40 20/80 60/40 Extrusion temp (° C.) 270 275 285280 280 280 Residence time (min) 6 8 15 10 8 12 Stretching method *2 *2*1 *1 *1 *1 Stretchability Good Good Good Good Good Good Film producingconditions MD temp(° C.) 55-60 55-60 80 80 90 80 Ratio 2.8 2.8 3 3 3 3TD temp(° C.) 80-90 80-90 80 80 90 80 Ratio 3.6 3.6 3.3 3.3 3.3 3.3 Heatsetting 150 150 150 150 150 150 temp(° C.) Thickness (μm) 25 25 25 25 2025

[0145] TABLE 2 Example 1 2 3 4 5 6 7 8 9 10 11 12 Stretched film Meltingpoint 221/252 221/251 222/251 222/247 214/251 218/239 221/253 220/252214/251 221/253 221/250 214/246 I/II (° C.) Intrinsic viscosity 0.760.92 0.99 0.94 0.77 0.81 0.76 0.92 0.77 0.86 0.91 0.94 Ester exchange 34 5 4 3 6 3 4 3 6 3 5 index (%) COOH-group 18 12 10 13 21 28 18 12 21 1715 26 (eq/ton) PBT residue 48 52 57 72 54 56 48 52 54 46 58 67 index (%)Tc (° C.) 72 70 70 67 68 69 72 70 68 71 68 65 Cp (J/g ° C.) 0.6 0.5 0.20.1 0.2 0.3 0.6 0.5 0.2 0.2 0.6 0.1 Elongation (%) 130 140 150 120 120130 130 150 130 140 130 120 in tension Laminated metal sheet Roll temp(° C.) 200 200 200 200 200 200 200 200 200 200 200 200 Heat- ⊚ ⊚ ◯ ⊚ ⊚ ⊚⊚ ⊚ ⊚ ⊚ ⊚ ⊚ laminatability Formability (mA) 0.8 0.4 0.5 0.8 1 0.7 0.70.5 1 0.5 0.3 0.8 Shock resistance ⊚ ⊚ ⊚ ⊚ ◯ ⊚ ⊚ ⊚ ◯ ⊚ ⊚ ⊚ (i) Shockresistance ◯ ⊚ ⊚ ◯ ◯ ◯ ◯ ⊚ ◯ ⊚ ⊚ ◯ (ii) Retort resistance ◯ ◯ ◯ ◯ ◯ ◯ ◯◯ ◯ ◯ ◯ ◯ Laminated can Taste and flavor ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯preservation Example Comparative Example 13 1 2 3 4 5 6 7 8 9 10 11Stretched film Melting point 222/234 215/247 222/245 221/252 211/237 242218/241 223 213/250 198/252 211/238 221/246 I/II (° C.) Intrinsicviscosity 0.93 0.86 0.86 0.67 0.71 0.78 0.85 0.75 0.72 0.8 0.68 0.86Ester exchange 4 7 5 4 10 — 5 4 6 4 9 5 index (%) COOH-group 19 23 20 1930 22 18 45 34 28 45 20 (eq/ton) PBT residue 58 31 82 47 54 0 27 58 5241 50 82 index (%) Tc (° C.) 61 72 62 70 75 185 76 67 69 65 69 62 Cp(J/g ° C.) 0.3 0.1 −0.3 0.2 −0.1 1.3 0.5 0.3 0.1 0.3 0.2 −0.3 Elongation(%) 130 130 70 110 130 140 130 130 120 120 90 60 in tension Laminatedmetal sheet Roll temp (° C.) 200 200 200 200 200 200 200 200 200 200 200200 Heat- ⊚ ◯ ⊚ ⊚ ⊚ ◯ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ laminatability Formability (mA) 0.50.8 X X X 1.2 0.8 0.9 2.4 X X X Shock resistance ⊚ Δ X X Δ X ◯ X ◯ X X X(i) Shock resistance ◯ X X X X X X X X X X X (ii) Retort resistance ◯ ◯◯ ◯ Δ Δ Δ X ◯ Δ ◯ ◯ Laminated can Taste and flavor ◯ ◯ Unaccept-Unaccept- Unaccept- X Δ X X Unaccept- Unaccept- Unaccept- preservationable able able able able able Comparative Example 12 13 14 15 16 17Stretched film Melting point 218/242 213/252 205 212/239 217/254 212/239I/II (° C.) Intrinsic viscosity 0.85 0.72 0.82 0.72 0.76 0.7 Esterexchange 5 6 15 9 5 11 index (%) COOH-group 18 34 35 29 18 31 (eq/ton)PBT residue 27 52 56 55 18 54 index (%) Tc (° C.) 76 69 75 67 120 72 Cp(J/g ° C.) 0.5 0.1 −2 0.1 1.2 −0.5 Elongation (%) 130 130 100 120 140110 in tension Laminated metal sheet Roll temp (° C.) 200 200 170 200230 200 Heat- ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ laminatability Formability (mA) 1.5 4.3 X X4.1 X Shock resistance Δ Δ X Δ X Δ (i) Shock resistance X X X Δ X Δ (ii)Retort resistance Δ ◯ ◯ ◯ Δ ◯ Laminated can Taste and flavor Δ XUnaccept- Unaccept- X Unaccept- preservation able able able

[0146] Examples 7 to 9 and Comparative Examples 11 to 13

[0147] Unstretched films as shown in Table 1 were each introduced into aroll-type vertical (MD) stretching machine, then preheated from 45° C.up to 55° C., and vertically stretched at a draw ratio of 2.8 at 55 to60° C. After being cooled, the resulting films were each continuouslyintroduced into a tenter-type transverse stretching machine, thenpreheated at 75° C. with opposite edges thereof being held by clips, andtransversely stretched at a draw ratio of 3.6 while being graduallyheated from 80° C. to 90° C. Thereafter, the resulting films were eachsubjected to a heat treatment at 150° C. for four seconds and then to a4% relaxation process, then cooled, and wound up. Thus, successivelybiaxially stretched films having a thickness of 25 μm were obtained.

[0148] With the use of each of the films thus obtained, a laminatedmetal sheet was produced and evaluated in the same manner as inExample 1. The physical properties of the films and the results of theevaluations are shown in Table 2.

[0149] The films of Examples 1 to 9 were excellent in heatlaminatability, formability, shock resistance, retort resistance, andtaste and flavor preserving property. In contrast, none of the films ofComparative Examples 1 to 13 are satisfactory in all the physicalproperties.

[0150] Comparative Example 14

[0151] As shown in Table 1, 65 parts by weight of the polyester (A-3)and 35 parts by weight of the polyester (B-2) were dry-blended, andmelt-extruded into a sheet form at 285° C. by means of an extruderhaving a T-die (residence time: 15 minutes). The resulting sheet wascooled in intimate contact with a cooling drum having a surfacetemperature of 18° C. Thus, a 240-μm thick unstretched sheet wasobtained.

[0152] The unstretched sheet thus obtained was transported through apreheating zone at 60° C. with edges thereof being held by clips of atenter-type simultaneous biaxial stretching machine, and thensimultaneously biaxially stretched at an MD draw ratio of 3.0 and at aTD draw ratio of 3.3 at a temperature of 80° C. Thereafter, theresulting sheet was subjected to a heat treatment at a TD relaxing ratioof 5% at a temperature of 150° C. for four seconds, then cooled to aroom temperature, and wound up. Thus, a biaxially stretched film havinga thickness of 25 μm was obtained.

[0153] The physical property values of the film thus obtained are shownin Table 2.

[0154] Comparative Examples 15 and 16

[0155] Films were each produced in substantially the same manner as inComparative Example 14, except that the types and blend ratios of thematerial resins and the film producing conditions were changed as shownin Table 1.

[0156] The physical properties of the films thus obtained are shown inTable 2.

[0157] None of the films obtained in Comparative Examples 14 to 16 weresatisfactory in all of the heat laminatability, the formability, theshock resistance, the retort resistance, and the taste and flavorpreserving property.

[0158] Example 10

[0159] First, 50 parts by weight of the polyester (A-3) and 50 parts byweight of the polyester (B-2) were dry-blended, and melt-extruded into asheet form at 275° C. by means of an extruder having a T-die (residencetime: 8 minutes). The sheet was cooled in intimate contact with acooling drum having a surface temperature of 18° C. Thus, a 240-μm thickunstretched sheet was obtained.

[0160] The unstretched sheet thus obtained was transported through apreheating zone at 60° C. with edges thereof being held by clips of atenter-type simultaneous biaxial stretching machine, and thensimultaneously biaxially stretched at an MD draw ratio of 3.0 and at aTD draw ratio of 3.3 at a temperature of 80° C. Thereafter, theresulting sheet was subjected to a heat treatment at a TD relaxing ratioof 5% at a temperature of 150° C. for four seconds, then cooled to aroom temperature, and wound up. Thus, a biaxially stretched film havinga thickness of 25 μm was obtained.

[0161] The film producing conditions and the physical property values ofthe film thus obtained are shown in Tables 1 and 2.

[0162] Examples 11 to 13 and Comparative Example 17

[0163] Films were each produced in substantially the same manner as inExample 10, except that the material resins, the blend ratios and thefilm producing conditions were changed as shown in Table 1.

[0164] The film producing conditions and the physical property values ofthe films are shown in Tables 1 and 2.

[0165] The films obtained in Examples 10 to 13 were excellent in heatlaminatability, formability, shock resistance and retort resistance. Incontrast, the film of Comparative Example 17 was unsatisfactory inproperties other than the retort resistance.

1. A polyester film for metal sheet laminating, the polyester filmcomprising a blend of a polyester (I) consisting of polybutyleneterephthalate or consisting essentially of polybutylene terephthalateand a polyester (II) consisting of polyethylene terephthalate orconsisting essentially of polyethylene terephthalate, the polyester (I)being present in the film in a proportion of 80 to 40 wt %, thepolyester (II) being present in the film in a proportion of 20 to 60 wt%, the polyester (I) having a melting point of 200 to 223° C., thepolyester (II) having a melting point of 230 to 256° C., the filmentirely having an intrinsic viscosity of not lower than 0.75.
 2. Apolyester film for metal sheet laminating, as set forth in claim 1,wherein an index of ester exchange between the polyester (I) and thepolyester (II) is 1 to 10 %.
 3. A polyester film for metal sheetlaminating, as set forth in claim 2, wherein the index of the esterexchange between the polyester (I) and the polyester (II) is 2 to 7% .4. A polyester film for metal sheet laminating, as set forth in any oneof claims 1 to 3, wherein terminal carboxyl groups are present in aproportion of not greater than 30 equivalents/ton.
 5. A polyester filmfor metal sheet laminating, as set forth in any one of claims 1 to 4,which has a polybutylene terephthalate residue index of 40 to 75%.
 6. Apolyester film for metal sheet laminating, as set forth in any one ofclaims 1 to 5, which has a heat-up crystallization peak temperature of60 to 100° C. at which a transition from an amorphous phase occurs.
 7. Apolyester film for metal sheet laminating, as set forth in any one ofclaims 1 to 6, which has a heat-up crystallization index of not smallerthan 0 J/g•° C. in a temperature range of 60 to 100° C.
 8. A polyesterfilm for metal sheet laminating, as set forth in any one of claims 1 to7, which has an elongation of not smaller than 100% at rupture at aheat-up crystallization peak temperature.
 9. A film-laminated metalsheet, where in a polyester film as recited in any one of claims 1 to 8is laminated on a metal sheet directly or with the intervention of anadhesive.
 10. A metal container produced by working a film-laminatedmetal sheet as recited in claim 9.